Method and System for User Equipment Location Determination on a Wireless Transmission System

ABSTRACT

Neighbor cell hearability can be improved by including an additional reference signal that can be detected at a low sensitivity and a low signal-to-noise ratio, by introducing non-unity frequency reuse for the signals used for a time difference of arrival (TDOA) measurement, e.g., orthogonality of signals transmitted from the serving cell sites and the various neighbor cell sites. The new reference signal, called the TDOA-RS, is proposed to improve the hearability of neighbor cells in a cellular network that deploys 3GPP EUTRAN (LTE) system, and the TDOA-RS can be transmitted in any resource blocks (RB) for PDSCH and/or MBSFN subframe, regardless of whether the latter is on a carrier supporting both PMCH and PDSCH or not. Besides the additional TDOA-RS reference signal, an additional synchronization signal (TDOA-sync) may also be included to improve the hearability of neighbor cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/241,601, filed Aug. 19, 2016, entitled “METHOD AND SYSTEM FOR USEREQUIPMENT LOCATION DETERMINATION ON A WIRELESS TRANSMISSION SYSTEM”,which is a continuation of U.S. patent application Ser. No. 14/509,379,filed Oct. 8, 2014, entitled “METHOD AND SYSTEM FOR USER EQUIPMENTLOCATION DETERMINATION ON A WIRELESS TRANSMISSION SYSTEM”, (now U.S.Pat. No. 9,423,488), which is a continuation of U.S. patent applicationSer. No. 13/147,272, filed Aug. 1, 2011, entitled “METHOD AND SYSTEM FORUSER EQUIPMENT LOCATION DETERMINATION ON A WIRELESS TRANSMISSIONSYSTEM,” (now U.S. Pat. No. 8,885,581), which is a Submission Under 35U.S.C. §371 for U.S. National Stage Patent Application of InternationalApplication No. PCT/US10/00446, of the same title, filed Feb. 5, 2010,which claims priority to U.S. Provisional Application Ser. No.61/174,333, filed Apr. 30, 2009, U.S. Provisional Application Ser. No.61/168,087, filed Apr. 9, 2009, and U.S. Provisional Application Ser.No. 61/150,137, filed Feb. 5, 2009; the disclosures of each of theabove-referenced applications are incorporated by reference in theirentireties as though fully and completely set forth herein.

The claims in the instant application are different than those of theparent application or other related applications. The Applicanttherefore rescinds any disclaimer of claim scope made in the parentapplication or any predecessor application in relation to the instantapplication. The Examiner is therefore advised that any such previousdisclaimer and the cited references that it was made to avoid, may needto be revisited. Further, any disclaimer made in the instant applicationshould not be read into or against the parent application or otherrelated applications.

TECHNICAL FIELD OF THE INVENTION

This application relates to wireless communication techniques ingeneral, and to determining position of user equipment using positioningreference signals in particular.

BACKGROUND OF THE INVENTION

There is an increasing demand on mobile wireless operators to providevoice and high-speed data services, and at the same time, mobile networkoperators wish to support more users per basestation to reduce overallnetwork costs and make services more affordable to subscribers. As aresult, wireless systems that enable higher data rates and highercapacities to the user equipment are needed. The available spectrum forwireless services is limited, however, and the prior attempts toincrease traffic within a fixed bandwidth have increased interference inthe system and degraded signal quality.

Various schemes have been implemented on orthogonal frequency domainmultiple access (OFDMA) systems to increase system performance.Technologies like multiple input multiple output (MIMO), orthogonalfrequency division multiplexing (OFDM), and advanced error control codesenhance per-link throughput, but these technologies do not solve allproblems encountered in the communication network.

Wireless communications networks are typically divided into cells, witheach of the cells further divided into cell sectors. A base stationtransceiver unit is provided in each cell to enable wirelesscommunications with mobile stations located within the cell sitelocation. Reference signals are transmitted from cell site base stationtransceivers on the cell site where the user equipment is located(eNodeB or serving cell sites), as well as being transmitted from basestation transceivers on various neighbor cell sites (neighbor cellsites) located around the serving cell site.

Reference signals are used by user equipment on an OrthogonalFrequency-Division Multiple Access (OFDMA) system, such as a 3GPP andLTE mobile wireless communication systems, to assist in establishing thelocation of user equipment on the mobile wireless communication system.In one form of location analysis, the user equipment uses the referencesignals received from the serving and neighboring cell sites todetermine the user equipment location to determine a time difference ofarrival between reference signals transmitted from the serving cell siteand the neighbor cell sites. By calculating a time difference of arrivalfor the reference signals, the user equipment or other components on thenetwork can perform a triangulation calculation to accurately determinethe location of the user equipment on the network. That locationinformation is used to adjust the power of transmission signals to andfrom the user equipment so as to reduce interference with other signalson the network and improve the overall accuracy of the signaltransmissions to and from the user equipment.

Neighbor cell hearability is the ability of the user equipment todetect, or “hear,” reference signals from neighbor cell sites. Referencesignals from the serving cell sites and neighboring cell sites, however,must be accurately detected, or “heard,” by user equipment in order tobe used in the location analysis. One problem encountered in neighborcell hearability arises when user equipment is located near to thecenter of the serving cell site such that reference signals fromneighbor cell sites are too weak for proper detection by the userequipment. In this situation, the reference signals from neighbor cellsites is too weak for the user equipment to accurately estimate the timedifference of arrivals between the reference signal from the serving andvarious neighbor cell sites, which hinders the triangulation locationanalysis conducted by the user equipment.

Known prior art systems and proposals do not adequately address theneighbor cell hearability problem that arises when user equipment islocated near to the center of the serving cell site, and these knownsystems and proposals include the following: (1) 3GPP TS 36.133 v8.4.0,“E-UTRA Requirements for support of radio resource management,” (2) 3GPPTS 36.214 v8.5.0, ‘E-UTRA; Physical layer measurements’, December 2008,(3) 3GPP TS 36.211 v8.5.0, ‘E-UTRA: Physical channels and modulation’,December 2008, (4) R1-090053, ‘Improving the hearability of LTEPositioning Service’, Alcatel-Lucent, RAN155bis, Ljubljana, Slovenia,January 2009, [1] (5) R1-090321, ‘Positioning Support for LTE Rel—9-RANISpecific Issues’, Motorola, RAN155bis, Ljubljana, Slovenia, January2009, and (6) R1-090353, ‘On OTDOA in LTE’, Qualcomm Europe, RAN1-55bis,Ljubljana, Slovenia, January 2009 [3].

In reference (4) and (6) identified above, different additionalreference signal patterns are proposed, but both these proposals do notprovide a workable or improved solution for the neighbor cellhearability problem when user equipment is located near the serving cellsite.

In reference proposal (4) identified above, one resource block (RB) fortransmitting a new reference signal RS pattern, called the LCS-RS, mustbe scheduled. However, the joint scheduling of the resource block RB fortransmitting the reference signal (LCS-RS) requires coordination betweenvarious neighbor cell sites which is not currently supported by networkcommunication systems. Further, reference proposal (4) above requiresthe cell sites to be synchronous, the new reference signal LCS-RSpattern has a different structure as compared to the cell-specificreference signal RS, called the CRS that is defined in the currentspecification. Lastly, collisions between clusters of neighbor cells maystill arise unless coordination is done extensively over a largercluster of the network. In order to implement this proposal identifiedby reference (4) above, a new type of reference signal is required thatis not recognized by the current network systems and large scalesynchronous coordination of system components would need to becoordinated. This proposal, therefore, requires changes to the existingsystem that are too extensive to be workable or practical.

With respect to the reference proposal (6) identified above, theproposed reference signal (E-IRDL RS) follows a very different structureas compared to that of the cell-specific reference signal (CRS) in theexisting standard, which requires the introduction of new, and complex,technology in the receivers. In order to implement this proposalidentified by reference (6) above, a new type of reference signal isrequired that is not recognized by the current network systems and theimplementation of new technology in receivers would be required. Thisproposal, therefore, also requires changes to the existing system thatare too extensive to be workable or practical.

Simulations were also conducted on the reference proposals (4), (5) and(6) identified above in a multi-cell, multi-sector deployment scenario,with user equipment simulated to be located with uniform randomness inthe serving cell site. 3GPP simulation results for Cases 1 and 3 areshown below with an FDD intra-frequency measurement sensitivityrequirement is set to be SGH_RP{tilde over ( )}126 dBm as defined in3GPP TS 36.133 v8.4.0. “E-UTRA Requirements for support of radioresource management.”

In the simulations, reuse mechanisms could be achieved in frequency,time, and/or code domain, but no specific reuse mechanism has beenassumed. The simulations did, however, assume reuse factors of 1, 3 and6. The Gil distributions of best N neighbor cell signals as observed byeach UE are captured and plotted as shown in FIGS. 1-3. The geometry(Gil) distribution of the signal from the serving cell is also plottedfor comparison. The cell hearability requirement defined in 3GPP TS36.133 v8.4.0, “E-UTRA Requirements for support of radio resourcemanagement” is SCH E={tilde over ( )}6 dB. In the present simulationstudy, hearability C/I requirements are assumed to be −6, −8 or −10 dB.The distribution of the number of neighbor cells with detectable signalare plotted as shown in FIGS. 4-6.

From the simulation data taken, the applicants observed the following.

-   -   For a Reuse Factor of 1, the probability that a UE can detect 3        or more sites is less than 20%, even when ISO=500 m (Case 1) the        hearability CII threshold is as low as −10 dB;    -   For a Reuse Factor of 3:        -   In Case 1, UE can detect 3 or more sites with probability of            about 69% when CII threshold is −6 dB, 77% at −8 dB, and 85%            at −10 dB;        -   In Case 3, UE can detect 3 or more sites with probability of            about 48% when CII threshold is −6 dB, 62% at −8 dB, and 73%            at −10 dB;    -   For a Reuse Factor of 6′        -   In Case 1. UE can detect 3 or more sites with probability of            about 98%, when CII threshold is −6 dB, and,        -   In Case 3, UE can detect 3 or more sites with probability of            about 77% when CII threshold is −6 dB, 86% at −8 dB, and 92%            at −10 dB.

Improving the accuracy in the calculation of the time difference of theserving cell site reference signal and the neighboring cell site willresult in an improvement in the accuracy of the location determinations,which will result in enhanced system performance and a reduction of lostdata and control signals to and from the user equipment. Increasing theaccuracy of the triangulation calculation without requiring extensivesystem changes or requiring wholesale changes to the reference band orreference signals is needed. Put another way, an improvement in theaccuracy of the user equipment positioning analysis when the userequipment is located near the serving cell site is needed, where theimprovement attempts to work within the constraints of the existingdeployed 3GPP and LTE system and without requiring extensive systemchanges or new hardware deployment. Based on simulation analysis andcomparative studies done on the existing systems and proposals, there isa need to improve the positioning-assisting reference signals so moreaccurate user equipment positioning can be achieved.

The various components on the system may be called different namesdepending on the nomenclature used on any particular networkconfiguration or communication system. For instance, “user equipment”encompasses PC's on a cabled network, as well as other types ofequipment coupled by wireless connectivity directly to the cellularnetwork as can be experienced by various makes and models of mobileterminals (“cell phones”) having various features and functionality,such as Internet access, e-mail, messaging services, and the like.

Further, the words “receiver” and “transmitter” may be referred to as“access point” (AP), “basestation,” and “user” depending on whichdirection the communication is being transmitted and received. Forexample, an access point AP or a basestation (eNodeB or eNB) is thetransmitter and a user is the receiver for downlink environments,whereas an access point AP or a basestation (eNodeB or eNB) is thereceiver and a user is the transmitter for uplink environments. Theseterms (such as transmitter or receiver) are not meant to berestrictively defined, but could include various mobile communicationunits or transmission devices located on the network.

SUMMARY OF THE INVENTION

Neighbor cell hearability can be improved by including an additionalreference signal that can be detected at a low sensitivity and a lowsignal-to-noise ratio, by introducing non-unity frequency reuse for thesignals used for a time difference of arrival (TDOA) measurement, e.g.,orthogonality of signals transmitted from the serving cell sites and thevarious neighbor cell sites. The new reference signal, called theTDOA-RS, is proposed to improve the hearability of neighbor cells in acellular network that deploys 3GPP EUTRAN (LTE) system, and the TDOA-RScan be transmitted in any resource blocks (RB) for PDSCH and/or MBSFNsubframe, regardless of whether the latter is on a carrier supportingboth PMCH and PDSCH or not.

Besides the additional TDOA-RS reference signal, an additionalsynchronization signal (TDOA-sync) may also be included to improve thehearability of neighbor cells. This modified or new TDOA-sync signal canbe transmitted in the OFDM symbols sharing the same resource blocks RBsas the synchronization channel. To increase the orthogonality, differentcell sites may use different OFOM symbols to transmit this TDOA-syncsignal.

The synchronization signals can also be extended (TDOA-sync) to maintainorthogonality between cell sites, with orthogonal or low correlationproperty through the primary and secondary synchronization signals asdefined in Release-8 standards, 3GPP TS 36.211v8.5.0. The resourceblocks (RB) carrying these additional signals can be transmitted byhopping through different frequency resources between subsequenttransmission instances. Alternatively, they can also hop within theresource blocks used for synchronization signals, i.e., when they aretransmitted in the same resource blocks RBs as the synchronizationchannel.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present application will now be described, by way ofexample only, with reference to the accompanying drawing figures,wherein:

FIG. 1-6 are charts showing performance characteristics based onsimulation results.

FIG. 7-9G are schematics diagrams of block assignments in a transmissionsignal;

FIG. 10 through 12 are network component diagrams for components on thecommunications network.

Like reference numerals are used in different figures to denote similarelements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 8, the block diagram shows a base station controller(BSC) 10 which controls wireless communications within multiple cells12, which cells are served by corresponding basestations (BS) 14. Insome configurations, each cell is further divided into multiple sectors13 or zones. In general, each base station 14 facilitates communicationsusing OFDM with mobile and/or wireless terminals 16, which are withinthe cell 12 associated with the corresponding base station 14. Themovement of the mobile terminals 16 in relation to the base stations 14results in significant fluctuation in channel conditions.

As illustrated, the base stations 14 and mobile terminals 16 may includemultiple antennas to provide spatial diversity for communications. Insome configurations, relay stations 15 may assist in communicationsbetween base stations 14 and wireless terminals 16. Wireless terminals16 can be handed from any cell 12, sector 13 zone, base station 14 orrelay 15 to another cell 12, sector 13 zone, base station 14 or relay15. In some configurations, base stations 14 communicate with each andwith another network (such as a core network or the internet) over abackhaul network 11.

The hearability problem solved by the present invention occurs when themobile terminal MS 16 is located in close proximity to the basestationtransceiver unit BS 14. Without the use the present invention, themobile terminal MS 16 would encounter problems with its triangulationanalysis, which would lead to problems with providing accurate locationor proximity data to the system. The invention solves this hearabilityproblem by using a reference signal (TDOA-RS) and an additionalreference synchronization signal (TDOA-sync). Neighbor cell hearabilitycan be improved by including an additional reference signal that can bedetected at a low sensitivity and a low signal-to-noise ratio, byintroducing non-unity frequency reuse for the signals used for a timedifference of arrival (TDOA) measurement, e.g., orthogonality of signalstransmitted from the serving cell sites and the various neighbor cellsites. The new reference signal, called the TDOA-RS, is proposed toimprove the hearability of neighbor cells in a cellular network thatdeploys 3GPP EUTRAN (L TE) system, and the TDOA-RS can be transmitted inany resource blocks (RB) for PDSCH and/or MBSFN subframe, regardless ofwhether the latter is on a carrier supporting both PMCH and PDSCH ornot.

An additional synchronization signal (TDOA-sync) may also be included toimprove the hearability of neighbor cells. This TDOA-sync signal can betransmitted in the OFDM symbols sharing the same resource blocks RBs asthe synchronization channel. To increase the orthogonality, differentcell sites may use different OFOM symbols to transmit this TDOA-syncsignal. The synchronization signals can also be extended (TDOA-sync) tomaintain orthogonality between cell sites, with orthogonal or lowcorrelation property through the primary and secondary synchronizationsignals as defined in Release-8 standards, 3GPP TS 36.211v8.5.0. Theresource blocks (RB) carrying these additional signals can betransmitted by hopping through different frequency resources betweensubsequent transmission instances. Alternatively, they can also hopwithin the resource blocks used for synchronization signals, i.e., whenthey are transmitted in the same resource blocks RBs as thesynchronization channel.

With reference to FIG. 9, an example of a base station 14 isillustrated. The base station 14 generally includes a control system 20,a baseband processor 22, transmit circuitry 24, receive circuitry 26,multiple antennas 28, and a network interface 30. The receive circuitry26 receives radio frequency signals bearing information from one or moreremote transmitters provided by mobile terminals 16 (illustrated in FIG.3) and relay stations 15 (illustrated in FIG. 4). In addition to thecomponents shown in FIG. 9, a low noise amplifier and a filter maycooperate to amplify and remove broadband interference from the signalfor processing. Further, downconversion and digitization circuitry willthen downconvert the filtered, received signal to an intermediate orbaseband frequency signal, which is then digitized into one or moredigital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (DSPs) orapplication-specific integrated circuits (ASICs). The receivedinformation is then sent across a wireless network via the networkinterface 30 or transmitted to another mobile terminal 16 serviced bythe base station 14, either directly or with the assistance of a relay15.

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, and encodesthe data for transmission. The encoded data is output to the transmitcircuitry 24, where it is modulated by one or more carrier signalshaving a desired transmit frequency or frequencies. A power amplifiermay also be used will amplify the modulated carrier signals to a levelappropriate for transmission, and deliver the modulated carrier signalsto the antennas 28 through a matching network (not shown). Modulationand processing details are described in greater detail below.

With reference to FIG. 10, an example of a user equipment or mobileterminal 16 is illustrated. Similar to the base station 14, the mobileterminal 16 will include a control system 32, a baseband processor 34,transmit circuitry 36, receive circuitry 38, multiple antennas 40, anduser interface circuitry 42. The receive circuitry 38 receives radiofrequency signals bearing information from one or more base stations 14and relays 15. A low noise amplifier and a filter may cooperate toamplify and remove broadband interference from the signal forprocessing. Downconversion and digitization circuitry will thendownconvert the filtered, received signal to an intermediate or basebandfrequency signal, which is then digitized into one or more digitalstreams.

The baseband processor 34 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. The baseband processor 34 is generallyimplemented in one or more digital signal processors (DSPs) andapplication specific integrated circuits (ASICs).

For transmission, the baseband processor 34 receives digitized data,which may represent voice, video, data, or control information, from thecontrol system 32, which it encodes for transmission. The encoded datais output to the transmit circuitry 36, where it is used by a modulatorto modulate one or more signals that is at a desired transmit frequencyor frequencies. A power amplifier can also be used to amplify themodulated carrier signals to a level appropriate for transmission, anddeliver the modulated carrier signal to the antennas 40 through amatching network.

Various modulation and processing techniques available to those skilledin the art are used for signal transmission between the mobile terminaland the base station, either directly or via the relay station. In OFDMmodulation, the transmission band is divided into multiple, orthogonalcarrier waves. Each carrier wave is modulated according to the digitaldata to be transmitted. Because OFDM divides the transmission band intomultiple carriers, the bandwidth per carrier decreases and themodulation time per carrier increases. Since the multiple carriers aretransmitted in parallel, the transmission rate for the digital data, orsymbols, on any given carrier is lower than when a single carrier isused.

Reference signals are used by user equipment, such as mobile terminal MS16, on an Orthogonal Frequency-Division Multiple Access (OFDMA) system,such as a 3GPP and LTE mobile wireless communication systems, to assistin establishing the location of user equipment on the mobile wirelesscommunication system. As shown in FIG. 10 and using one form of locationanalysis, the user equipment MS 16A uses the reference signals receivedfrom the serving cell site controller BS 14A and neighboring cell sitesBS 14B and/or 14C to determine the user equipment location based on atime difference of arrival analysis using the time difference referencesignals transmitted from the serving cell site BS 14A and the neighborcell sites BS 14B and/or 14C.

By calculating a time difference of arrival for the reference signals,the user equipment MS 16A or other components on the network can performa triangulation calculation to accurately determine the location of theuser equipment MS 16A on the network That location information is usedto adjust the power of transmission signals to and from the userequipment MS 16A so as to reduce interference with other signals on thenetwork and improve the overall accuracy of the signal transmissions toand from the user equipment.

Neighbor cell hearability is the ability of the user equipment todetect, or “hear,” reference signals from neighbor cell sites BS 14B or14C. Reference signals from the serving cell sites BS 14A andneighboring cell sites BS 14B or 14C, however, must be accuratelydetected, or “heard,” by user equipment MS 16A in order to be used inthe location analysis. One problem encountered in neighbor cellhearability arises when user equipment MS 16A is located near to thecenter of the serving cell site controller BS 14A such that referencesignals from neighbor cell sites BS 14B or 14C are too weak for properdetection by the user equipment. In this situation, the referencesignals from neighbor cell sites BS 14B and 14C is too weak for the userequipment to accurately estimate the time difference of arrivals betweenthe reference signal from the serving BS 14A and various neighbor cellsites BS 14B and/or 14C, which hinders the triangulation locationanalysis conducted by the user equipment MS 16A.

Neighbor cell hearability can be improved by including an additionalreference signal that can be detected by user equipment MS 16A at a lowsensitivity and a low signal-to-noise ratio, by introducing non-unityfrequency reuse for the signals used for a time difference of arrival(TDOA) measurement, e.g., orthogonality of signals transmitted from theserving cell sites and the various neighbor cell sites. The newreference signal, called the TDOA-RS, is proposed to improve thehearability of neighbor cells BS 14B and 14C in a cellular network thatdeploys 3GPP EUTRAN (LTE) system, and the TDOA-RS can be transmitted inany resource blocks (RB) for PDSCH and/or MBSFN subframe, regardless ofwhether the latter is on a carrier supporting both PMCH and PDSCH ornot.

Besides the additional TDOA-RS reference signal, an additionalsynchronization signal (TDOA-sync) may also be included to improve thehearability of transmissions from neighbor cells sites BS 14B and 14 c.Primary or secondary synchronization signals can be covered or scrambledby a cell-specific orthogonal code such as a Walsh code or other codeswith low cross-correlation property. If Walsh code is used, all 1'scodewords are reserved in the normal primary or secondary signal. Thescrambling is performed on the synchronization sequence before mappingto the resource elements for 10FT processing Other sequences withorthogonal or low cross-correlation property with the primary orsecondary synchronization signals can also be used.

This new TDOA-sync signal can be transmitted in the OFOM symbols sharingthe same RBs as the synchronization channel. To increase theorthogonality, different cell sites may use different OFOM symbols totransmit this TDOA-sync signal. For example, depending on the cell 10,the TDOA-sync signal can be transmitted in OFOM symbol e=2, 3, 9, 10, 12or 13 respectively, in the case of normal CP, Frame structure 1. OFOMsymbol e=0, 1, 4, 7 and 11 are reserved for cell-specific RS, while e=5,6 are reserved for the secondary and primary sync signals respectivelyin subframe 5. For slot 1 in subframe 0, the TDOA-sync signal cannot besent in OFOM symbol e=0, 1 . . . 3 to avoid collision with the broadcastchannel PBCH, if it is sent in the same resource block RB.

The synchronization signals can also be extended (TDOA-sync) to maintainorthogonality between cell sites, with orthogonal or low correlationproperty through the primary and secondary synchronization signals. Theresource blocks (RB) carrying these additional signals can betransmitted by hopping through different frequency resources betweensubsequent transmission instances. Alternatively, the TDOA-sync signalscan also hop within the resource blocks used for synchronizationsignals, i.e., when they are transmitted in the same resource blocks RBsas the synchronization channel.

For subframe #0, the TDOA-sync signals should be transmitted on the OFDMsymbols not used for broadcast channel PBCR transmissions. The group of6 resource blocks RBs carrying the primary and secondary synchronizationsignals and TDOA-sync can also hop to different frequency locations awayfrom the center of the carrier, in the interval between theslots/subframes specified in the current specification Ts 36.211 v8.5.0.This would also help to improve the synchronization performance forusers who may experience fading in the center of the band.

The TDOA-RS and TDOA-sync signals may also be transmitted on MBSFNsubframes on carriers with or without PDSCR support. Or, they can bescheduled jointly by neighbor cells for transmissions in the sameresource block RB as described in other proposals. The periodicity ofthe subframes carrying TDOA-RS and/or TDOA-sync is configurable,depending on the required TDOA estimation accuracy and the locationdistribution of the users (UEs or MS 16) in the cell site. Similarly,the number of resource blocks RBs carrying the TDOA-RS in one subframeis also configurable.

Similar to Idle period downlink transmission (IPDL) in UTRAN, groups ofresource blocks RBs in a certain subframe can be reserved for exclusivetransmissions by different neighboring cell sites BS 14B or BS 14C(except for the CRS signal) or in the data region of MBSFN subframes.The advantage for OFDMA-based EUTRAN (LTE) is that transmissions frommultiple neighbor cells BS 14B and BS 14C on these reserved resourceblocks RBs can be done simultaneously, as a form of fractional frequencyreuse within the subframe or group of subframes for TDOA measurement.These transmissions can be power boosted, with or without additionalreference signals. A fractional frequency reuse scheme could be appliedto further improve the hearing performance. For example, a special zonecan be reserved for the transmission of the additional cell specificsequence where different frequency reuse factors can be configured amongthe neighboring cells.

We can apply the similar design to relay stations to allow them tomonitor their neighboring relay stations, because relay cannot listen(detect SCH from neighbor relay stations) while talking. In addition,the timing differences between different base stations (eNB) in anasynchronous network can be identified through inquiry and responsesbased on X2 signaling. The relative timing differences between differentbase stations can be used by the network entity for locationdetermination, e.g. LMU.

To maintain consistency with the cell-specific reference signal RS (CRS)as defined in current Release-8 standard, an alternative structure ofthe additional TDOA reference signal (TDOA-RS) for UE positioning isshown in FIG. 7, for the case of normal CP. The advantage of similarityin the structure of these TDOA-RS as compared to CRS is that the similarreceiver can be used for detecting TDOA-RS. Similar to cell-specific RS,these TDOA-RS are also cell-specific, with the amount of shift as afunction of cell 10. A major difference from the standard CRS signalingis that the TDOA-RS configured for antenna ports 1, 2 or 3 can be usedby antenna port 0, when there is only a single antenna port. Likewise,when there are two transmit antenna ports only, TDOA-RS for antennaports 2 and 3 can be used for antenna ports 0 and 1 respectively.TDOA-RS that are transmitted from different antenna ports can be eithercombined to increase the accuracy of the TDOA estimation.

The TDOA-RS can be transmitted in any resource blocks (RB) for PDSCHand/or MBSFN subframe, regardless of whether the latter is on a carriersupporting both PMCH and PDSCH or not. While the situation is similarfor TDOA-sync signals, there is an additional constraint that theTDOA-sync signal has to be transmitted over 6 consecutive RBs in asubframe. The TDOA-sync signal can share the same RB as the primary andsecondary synchronization signals. Alternatively, the TDOA-RS can alsobe transmitted in the RBs that are used for the synchronizationchannels, with some modifications as shown in FIG. 8.

To exploit frequency diversity gain and to ensure the maximum number ofUEs in the cell site can detect the TDOA-RS and TDOA-sync signals, theresource blocks that carry these signals are allowed to hop betweentransmission instances of the signals H-lopping across the entire bandFrequency diversity gain can be maximized by hopping across the entireband, according to a pre-determined cell-specific hopping sequence. Inthe case of TDOA-RS, one or a few contiguous RBs carrying the TDOA-RScan hop to different frequency resources between consecutive transmitinstances.

In the case of TDOA-sync, the group of 6 contiguous RBs carrying theTDOA-sync can hop to different frequency resources, e.g., a differentgroup of 6 contiguous RBs between consecutive transmit instances. In thecase of TDOA-RS that shares the same RB as the sync channel, somefrequency diversity gain can also be obtained through hopping acrossthose 6 RBs that carry the sync channel.

Depending on the signal sensitivity, TDOA estimation accuracyrequirement and UE location distributions in the cell site, theperiodicity for transmitting the TDOA-RS and TDOA-sync signals can beconfigured to transmit in each subframe for a higher signal density, orin the same subframe as the sync signal. Either or both using subframe 0and 5 in each radio frame.

In the extreme case where the reuse factor is relatively high, the wholeresource block RB may be used by one cell sites. Then the additionalreference signals can occupy the entire RB, except for the resourceelements used for CRS to maintain backward compatibility with Release-B.The reuse factor could be configured by network and be broadcasted by anew SIB message. If FFR is configured, the transmission region for eachcell can be determined based on cell ID. Frequency hopping can also beapplied on top of FFR, by including a pre-determined cell specifichopping pattern with the configuration message.

In UTRAN, the LMU is responsible to estimate the relative timing offsetbetween the neighbor cells by observing their transmissions. To avoidhearability problem for E-UTRAN at the LMU, an alternative way for theLMU to find out about the relative timing offset between the neighborcells in an asynchronous system is to have a designated eNB send aninquiry to the neighbor cells on their timing information. If a neighboreNBs are equipped with a satellite receiver for GPS or GNSS signals,then the neighbor eNB can respond with an absolute timing of the frameboundary, as an example. Otherwise, the neighbor eNB can respond withrelative timing information, e.g., time stamp when the inquiry isreceived from eNB, and the timing offset of the corresponding subframeand SFN at the neighbor eNB. This inquiry and response can betransmitted through X2 signaling. The periodicity of this inquiry willdepend on the expected reference clock drifts at the eNB.

To evaluate the performance of a positioning method such as TDOA, thefactors that affect the accuracy would need to be captured. Evaluationmodels should capture the accuracy of TDOA estimation by the UEs,depending on the corresponding location in the cell, e.g., the TDOAestimation error performance as a function of SINR should be plotted andcaptured in the system level simulation. System simulation is used toevaluate the resultant performance on the estimation of UE locationbased on triangulation using the TDOA estimations of signals transmittedfrom various neighbor eNBs

The present invention solves hearability problems by using a newreference signal (TDOA-RS) and a new synchronization signal (TDOA-sync)transmitted over a signal structure that is already in use on the EUTRAN3GPP Release 8 standard, i.e., cell-specific RS and synchronizationsignals. By using new signals over an existing signal structure, thepresent invention can be implemented without adding additional receivercomplexity in the support of TDOA estimation. Moreover, hopping of theresource blocks that carry the proposed TDOA-RS and TDOA-sync across thefrequency domain can exploit frequency diversity, and maximize thehearability in various channel conditions as experienced by differentUEs.

The present invention can be used to allow relay station to maintain theon-going measurement of the cell specific sequence sent by itsneighboring relay stations. Such measurement can help the scheduling ofthe base station and cannot rely on the original reference signal (CRS)as defined in Release-8, because the relay station also needs totransmit CRS, especially for synchronous network. This mechanism isuseful for the self-organized relay network where the relay station canbe added/removed dynamically or relay station is moving.

To support UE location determination through time difference of arrival(TDOA) measurements, additional UE measurement capability should bedefined as follows.

The Definition for timing offset measurement, in units of T is the timeof arrival of a downlink frame in the neighbor cell (TOA_neighbor) withreference to the time of arrival of the corresponding frame in theserving ceil (TOA_ref), i.e., TOA_neighbor—TOA_ref, where T, is thebasic time unit for E-UTRA, as defined in T8 36.211 vS.5.0 s11. Thismodification to the system would be applicable for RRC_CONNECTEDintra-frequency, RRC CONNECTED inter-frequency.

The reporting format and triggering mechanism of such a measurement willbe defined as part of the MAC or RRC layer specifications. Onetriggering mechanism is based on the triggering of RSRP and RSRQreporting, as some timing offset information can be made available. Inaddition, the triggering can be based on the configuration of TDOA-RSand TDOA-sync transmission time, for improved accuracy, especially forUEs located in the cell center. For UEs located near to the cell edge,the timing offset measurement can be reported at the same time as theRSRP and/or RSRQ reports.

Additional positioning reference signals (RS) are proposed which haveproperty of frequency/time/code reuse 6 or greater. This is expected togreatly improve the neighbor cell signal hearability by the UTE MS 16over known methods and systems, which is only supporting reuse-3 in asystem deployment with 2 transmit antennas.

Further details have also been provided for the contiguous RS which hasa similar structure to the synchronization signals Avoid interferencefrom neighbor cell transmissions by assigning resources for thepositioning RS (TDOA-RS) orthogonally, through time, frequency and code,with an effective reuse factor of 6 or higher. A frequency reuse of 3has been analyzed, but it is not sufficient to achieve sufficientneighbor cell hearability. Thus, the present invention allows afrequency reuse pattern of 6 in the new positioning-assisting referencesignal RS (TDOA-RS). Similar to cell-specific RS (CRS), there iscell-specific frequency shift in the RS pattern: V,shift=N(cell ID) mod6, as described in Section 6.10.1 of TS 36.211 v8.6.0

The sequences used for positioning RS can be similar to that used forthe CRS. Alternatively, other pseudo-random sequences may also be used,e.g., Zadoff-Chu sequences. When MBSFN subframes are used to transmitpositioning RS, the cell-specific RS is only transmitted in OFDM symbol0 of the subframe when one or two transmit antenna ports are configured.Thus, positioning RS can be transmitted in all other OFDM symbols in thesubframe. However, OFDM symbol I of the subframe cannot be used forpositioning RS when more than two transmit antenna ports are configuredfor some neighbor cells in the deployed network.

The reference signal TDOA-RS can be positioned in contiguous frequency(subcarriers) over a number of resource blocks. In the example as shownin the chart below, each OFDM symbol that has no cell-specific referencesignal RS, each spanning over the 6 contiguous resource block RBs, andthe TDOA-RS reference signal is allocated to a neighbor cell for thetransmission of a cell-specific sequence. These are transmitted on OFDMsymbols not used for the synchronization signals and not for thebroadcast channel. Data traffic cannot be scheduled on those resourceblocks RBs carrying the TDOA-RS positioning reference signal.

If these reference signals RS are transmitted in the same resourceblocks RBs and subframe (#5) as the synchronization signals, then therecan be at most Nrs=6 OFDM symbols for up to 6 different neighbor cellsin deployment networks with more than 2 transmit antennas, and Nrs=8OFDM symbols for up to 8 different neighbor cells in deployment networkswith one or two transmit antennas only. Each neighbor cell can beassigned more than one OFDM symbol for transmitting the positioningsequence. Other subframes and resource blocks that do not carry thesynchronization signals (PSS/SSS) may also be used for transmitting thepositioning sequence. A larger number of resource blocks RBs or a longersequence length Nseq can also be supported, e.g., 15 resource blocks RBsas in the 3 MHz bandwidth system with 3.84 MHz sampling rate.

These sequences are orthogonal or have low cross-correlation propertieswith the primary (PSS) and secondary (SSS) synchronization signals, andbetween different neighbor cells. One type of sequence that can be usedis the CAZAC sequences or Zadoff-Chu sequences that are already used forvarious reference signals RS. One example is to use different cyclicshifts α of the length-62 primary synchronization signal den) as definedin Release-8 of the 3GPP standard.

${p(n)} = {{e^{j\; \alpha}{d(n)}\mspace{14mu} {where}\mspace{14mu} {d_{u}(n)}} = \left\{ \begin{matrix}e^{{- j}\frac{{\pi {un}}{({n + 1})}}{63}} & {{n = 0},1,\ldots \mspace{14mu},30} \\e^{{- j}\frac{{{\pi u}{({n + 1})}}{({n + 2})}}{63}} & {{n = 31},32,\ldots \mspace{14mu},61}\end{matrix} \right.}$

The values of cyclic shifts a can be chosen such that they are spaced asfar apart as possible for the different neighbor cells. The amount ofshifts should be a function of the cell ID. Similarly, the OFDM symbolassigned to a neighbor cell should also be a function of the cell ID.For example,

α=N _(int) +N _(ID) ⁽¹⁾mod └(N _(seq) /N _(int))┘

where N>=1: minimum number of samples between each possible cyclic shiftvalues. For example, for Nseq=62, and 6 desirable distinct cyclic shiftvalues, then

${N_{int} \leq {\frac{62}{6}}} = 10$

OFDM symbol assigned to neighbor cell with cell ID:

l′=N_(ID) ^(cell) mod N_(rs)

Where l′=OFDM symbols not containing CRS, PSS/SSSIPBCH, arranged inascending order of OFDM symbol index l, starting from l′=0 to l′=Nrs−1.For a longer sequence, the ZC sequence of length 127 can be used.

${d_{u}(n)} = \left\{ {{{e^{{- j}\frac{{\pi {un}}{({n + 1})}}{127}}\mspace{14mu} n} = 0},1,\ldots \mspace{14mu},126} \right.$

The root of these sequences u can be selected different from the onesused for the shorter sequence or primary sync signal.

As shown in FIG. 9F, when positioning the TDOA-RS reference signal in acontiguous resource blocks, the TDOA-RS is located in the same subframeas synchronization signals. The left block uses the same length as syncsignals, and the right block uses a longer length of reference signal RSsequence that may occupy about 15 RBs in the center of the band. Theseresource blocks are identified for deployment in networks with more than2 antennas. An OFDM symbol index 1 in each slot should be reserved andnot used for the positioning RS transmission.

Positioning of the TDOA-RS reference signal in a staggered pattern andcontiguous pattern is beneficial, with a frequency reuse of 6 and theobjective for the staggered pattern. With the use of different OFDMsymbols for contiguous positioning of the TDOA-RS transmission withdifferent cyclic shifts from different neighbor cells, a frequency reusepattern larger than 6 can be achieved.

The TDOA-RS positioning reference signals have been described using afrequency/time/code reuse of 6 or greater. This code reuse is expectedto greatly improve the neighbor cell signal hearability by the UE, ascompared to that of cell-specific RS in Release-8, which is onlysupporting reuse-3 in a system deployment with 2 transmit antennas.Further details have also been provided for the contiguous RS which hasa similar structure to the synchronization signals.

The present invention will avoid interference from neighbor celltransmissions by assigning resources for the positioning referencesignal RS (TDOA-RS) orthogonally, through time, frequency and code, withan effective reuse factor of 6 or higher. During the simulationanalysis, it was assumed that the network is synchronized. The mobileterminal UE time difference of arrival was measured based on eitherRelease-8 CRS or proposed PA-RS in designated subframes without datatransmission, i.e., IPDL subframes. Furthermore, FDD intra-frequencymeasurement sensitivity requirement is set to be SCI RP>−126 dBm [6].Cell detectability requirement is SCH E/Iot>6 dB. Using Rel-8 CRS, it isassumed that signal can be reliably detected down to around −14 dB dueto higher CRS symbol density than that of sync signal.

From the simulation results, it is observed that using Release-8 CRSonly. UE positioning performance cannot meet the US FCC mandate E911phase-2 requirement. Positioning accuracy is essentially limited byneighboring cell hearability. For example, for C/me threshold of −14 dB,the saturation point of position error at 83% indicates that theprobability that DE cannot detect 3 or more non-co-located sites is 17%.

For the standard shown in Release-8 using a CRS signal, a reuse factorof 3 is achievable for 2 antenna ports in IPDL subframes, i.e.,subframes without scheduled data. In case of joint scheduling orconfiguration of positioning subframes consisting of a mixture of normaland MBSFN subframes for different cell/group of cells, however, a higherreuse factor can be achieved, but only at the expense of increasedoverhead and complexity. The UE positioning error distributions for 3GPPSimulation Case-1 (ETU 3 kmlh) and Case-2 (ETU 30 kmlh) using Release-8CRS with a reuse factor of 6 were analyzed, with a C/I threshold forcell hearability is set to be −6, −10, and −14 dB.

From the simulation results, it was also observed that using Release-8CRS with a reuse factor of 6, UE positioning performance can meet theFCC E911 phase-2 requirement. Furthermore, it can be seen thatpositioning performance is impacted by the CII threshold for cellhearability. To be specific, setting the C/I threshold to a very lowvalue results in more inaccurate time difference of arrival estimates,which degrades positioning performance. On the other hand, if C/Ithreshold is set too high, cell hearability will be reduced. Therefore,a C/I threshold can be used to select neighbor cells for inclusion in UEpositioning determination, to avoid the degradation in positioningaccuracy as caused by TDOA estimates with large errors. The CA thresholdneeds to be optimized for a trade-off between the TDOA estimationaccuracy and the number of neighbor cells used in the positioningdetermination, i.e., the subsequent trilateration step, from thepositioning performance point of view.

PA-RS patterns with a reuse factor of 6 were simulated to analyze the UEpositioning error distributions for 3GPP Simulation Case-1 (ETU 3 kmlh)and Case-2 (ETU 30 kmlh) using an additional position reference signal,such as the TDOA-RS or PA-RS reference signal. The simulation wasconducted to analyze the additional position reference signal use with1-Tx or 2-Tx antenna configurations and PA-RS bandwidths of 50, 25, 15and 6 RBs.

From the simulation results using an additional position referencesignal, such as the TDOA-RS or PA-RS, with a reuse factor of 6, it isobserved that: (1) UE positioning performance can meet the FCC E911phase-2 requirement with an additional position reference signal, suchas the TDOA-RS or PA-RS, in a bandwidth of 15 RBs and higher in bothCase-1 and Case-2, and (2) for a bandwidth of 50 REs, the antennaconfiguration used to transmit the additional position reference signal,such as the TDOA-RS or PA-RS, has negligible impact on positioningperformance With reduced bandwidth, 2-Tx PA-RS generally improvespositioning performance due to diversity.

Using a standard positioning algorithm was adopted to conduct thesimulation, and the UE position is determined as follows.

Channel impulse response was estimated from serving and detectableneighboring cells, and it was shown that: (1) if more than onedetectable cell are co-located, the one with the best signal quality isemployed in positioning, (2) when multiple transmit/receive antennas aredeployed, estimated channel taps of all transmit-receive antenna pairsare combined coherently, (3) when both CRS and an additional positionreference signal, such as the TDOA-RS or PA-RS are configured, estimatedchannel taps from CRS and PA-RS are first combined using time-domaininterpolation for each Tx antenna port transmitting both CRS and PA-RS.Estimated channel taps of all transmit-receive antenna pairs are thencombined coherently, and (4) First arrival tap (path) is identified asthe earliest tap in the set of the strongest Ntap taps.

Propagation delay of signal from a cell is determined as the delay ofthe identified first tap. The time difference of arrival is determinedas the difference between delay from neighboring cell and serving cell.

The UE position is estimated from the time difference of arrival of theNns for neighboring sites with the best signal quality. The number ofneighbor cell time difference of arrival measurements is limited by asignal quality threshold, i.e. C/I threshold such that TDOA measurementsthat are expected to have large errors are not used for positionestimation, provided that a minimum number, e.g., 2 or 3, of neighborcell time difference of arrival measurements are available. A differentC/I threshold value may be applicable for a different number of neighborcell, i.e., there can be multiple CII thresholds.

For example, if there are Nns=5 neighbor cells with C/I exceeding C/Ithreshold 1 for hearability, the value of C/I threshold 2 for accurateTDOA measurement can be set to a relative higher value than in the caseNns=2. The threshold value settings can be selected based on the TDOAlink performance, and the cell-specific C/I threshold is configured byeNB through higher-layer signaling. UE-specific C/I threshold adjustmentcan also be supported. In this simulation, we assume Ntap=6 and C/Ithresholds of −6, −10 and −14 dB.

Based on the applicant's simulation analysis, it is shown that thepresent invention out-performs the standard Release 8 UE positioning forcell-specific (CRS). The applicant was able to determine that theCRS-based solution has limited positioning performance due toneighboring cell hearability problems. When joint scheduling orconfiguration of positioning subframes consisting of a mixture of normaland MBSFN subframes among different cells/group of cells, the CRS-basedpositioning performance can be improved, but only with an increasedreuse factor that will increase complexity and overhead on the system.

Cell hearability threshold can be optimized to improve positioningperformance. The additional position reference signal, such as theTDOA-RS or PA-RS, design with a reuse factor of 6 provides significantlyimproved positioning performance over known techniques, and FCC E911phase-2 requirement can be met with additional position referencesignal, such as the TDOA-RS or PA-RS, bandwidth of as low as 15 resourceblocks RBs in a 2 transmit antennas system. The impact of the proposedadditional position reference signal, such as the TDOA-RS or PA-RS,antenna configuration was analyzed, and it was determined that a 2-Txantenna configuration for the additional position reference signal, suchas the TDOA-RS or PA-RS, improves positioning performance forpositioning subframes based on normal subframe with no data or MBSFNsubframe with no data and CRS in the data region.

It was also determined that the time difference of arrival estimationaccuracy increased as the bandwidth of the positioning assistedreference signal, TDOA-RS or PA-RS, was increased. The positioningassisted reference signal, TDOA-RS or PA-RS does not need a full band,even though the time difference of arrival estimation error is abouttwice as much with a half band as that for the case of a full-band. Theresulting positioning performance after trilateration can still meet FCCrequirements.

Two proposed additional position reference signal, such as the TDOA-RSor PA-RS, patterns are shown in FIG. 9G for one antenna and two antennaconfigurations, respectively.

The above-described embodiments of the present application are intendedto be examples only. Those of skill in the art may effect alterations,modifications and variations to the particular embodiments withoutdeparting from the scope of the application. In the foregoingdescription, numerous details are set forth to provide an understandingof the present invention. However, it will be understood by thoseskilled in the art that the present invention may be practiced withoutthese details. While the invention has been disclosed with respect to alimited number of embodiments, those skilled in the art will appreciatenumerous modifications and variations therefrom. It is intended that theappended claims cover such modifications and variations as fall withinthe true spirit and scope of the invention.

1.-23. (canceled)
 24. An apparatus, comprising: one or more processingelements configured to couple to a wireless radio; and one or morememories having program instructions stored thereon that are executableby the one or more processing elements to cause the apparatus to:receive a first time difference of arrival reference signal (TDOA-RS)from a first base station serving the apparatus via a first cell and asecond TDOA-RS via a second cell over a radio access network; receive acell specific reference signal via the first cell, wherein the cellspecific reference signal is different from the first TDOA-RS; determinea time difference of arrival between the first TDOA-RS received from thefirst base station serving the apparatus via the first cell and thesecond TDOA-RS received via the second cell over the radio accessnetwork; report the determined time difference; wherein the firstTDOA-RS is transmitted using one or more first resource elements in oneor more resource blocks that are configured for the first cell, whereinone or more second resource elements in the one or more resource blocksthat are configured for transmission of the second TDOA-RS via thesecond cell are not used by first cell to transmit the first TDOA-RS;wherein a resource element corresponds to one subcarrier in oneorthogonal frequency-division multiplexing (OFDM) symbol, wherein aresource block consists of a plurality of resource elements occupying acontiguous set of subcarrier frequencies across a plurality of OFDMsymbols, and wherein resource elements that are configured for ones ofthe first and second cells are based at least in part on respective cellidentifiers of the first and second cells; wherein the one or more firstresource elements in the one or more resource blocks used for the firstTDOA-RS correspond to a first transmission instance, wherein the firstand second TDOA-RS are transmitted in a plurality of transmissioninstances; and wherein frequency hopping is implemented for the firstTDOA-RS such that different sets of one or more resource blocks are usedfor the first TDOA-RS for consecutive transmission instances of thefirst TDOA-RS.
 25. The apparatus of claim 24, wherein a third cell isconfigured to transmit TDOA-RS via the radio access network, and whereinthe plurality of transmission instances include one or more groups ofresource blocks in at least one subframe that are reserved for TDOA-RStransmission for at least the first and second cells, wherein the thirdcell transmits cell-specific reference signals and does not transmitTDOA-RS using the reserved one or more groups of resource blocks. 26.The apparatus of claim 24, wherein the first and second TDOA-RS are nottransmitted using resource elements configured for cell-specificreference symbols.
 27. The apparatus of claim 24, wherein the apparatusis configured to report the determined time difference in response to atrigger that is based on radio resource control (RRC) signaling.
 28. Theapparatus of claim 24, wherein the apparatus is configured to determinea location of the apparatus based on the first and second TDOA-RS. 29.The apparatus of claim 24, wherein the first cell is a serving cell andthe second cell is a neighbor cell.
 30. The apparatus of claim 24,wherein the apparatus is configured to determine a set of one or moretime differences of arrival for one or more neighbor cells from whichthe apparatus determines a received TDOA-RS signal quality to be above aparticular threshold.
 31. An apparatus, comprising: one or moreprocessing elements configured to couple to a wireless radio; and one ormore memories having program instructions stored thereon that areexecutable by the one or more processing elements to cause the apparatusto: transmit a first time difference of arrival reference signal(TDOA-RS) to a mobile device via a radio access network using one ormore first resource elements that are configured for a first cell servedby the apparatus and not one or more second resource elements that areconfigured for transmission of a second TDOA-RS by a second cell,wherein a resource element corresponds to one subcarrier in oneorthogonal frequency-division multiplexing (OFDM) symbol, whereinresource elements that are configured for ones of the first and secondcells are based at least in part on respective cell identifiers of thefirst and second cells, and wherein the first and second TDOA-RS aretransmitted in a plurality of transmission instances; transmit acell-specific reference signal to the mobile device; utilize frequencyhopping for the first TDOA-RS such that different sets of one or moreresource blocks are used for the first TDOA-RS for consecutivetransmission instances of the first TDOA-RS; and receive a report of adetermined time difference of arrival between the first TDOA-RS and thesecond TDOA-RS from the mobile device.
 32. The apparatus of claim 31,wherein the one or more first resource elements used for the firstTDOA-RS are in a first group of resource blocks in one or more subframesthat are reserved for TDOA-RS transmission for at least the first andsecond cells and are not transmitted in a second group of resourceblocks in one or more subframes that are reserved for TDOA-RStransmission for at least a third cell, wherein the resource blocks eachconsist of a plurality of resource elements occupying a contiguous setof subcarrier frequencies.
 33. The apparatus of claim 31, wherein thefirst and second TDOA-RS are not transmitted using resource elementsconfigured for cell-specific reference symbols.
 34. The apparatus ofclaim 31, wherein the report of the determined time different of arrivalis received in response to a trigger that is based on radio resourcecontrol (RRC) signaling.
 35. The apparatus of claim 31, wherein theapparatus is included in a serving base station for the mobile device.36. An apparatus, comprising: one or more processing elements configuredto couple to a wireless radio; and one or more memories having programinstructions stored thereon that are executable by the one or moreprocessing elements to cause the apparatus to: receive a first timedifference of arrival reference signal (TDOA-RS) received from a firstbase station serving the apparatus via a first cell and a second TDOA-RSreceived via a second cell over a radio access network, wherein a thirdcell is configured to transmit TDOA-RS via the radio access network;receive a cell specific reference signal via the first cell differentfrom the first TDOA-RS; determine a time difference of arrival betweenthe first TDOA-RS received from the first base station serving theapparatus via the first cell and the second TDOA-RS received via thesecond cell over the radio access network; report the determined timedifference; wherein the first TDOA-RS is transmitted using one or morefirst resource elements in one or more resource blocks that areconfigured for the first cell, wherein one or more second resourceelements in the one or more resource blocks that are configured fortransmission of the second TDOA-RS via the second cell are not used byfirst cell to transmit the first TDOA-RS; wherein a resource elementcorresponds to one subcarrier in one orthogonal frequency-divisionmultiplexing (OFDM) symbol, wherein a resource block consists of aplurality of resource elements occupying a contiguous set of subcarrierfrequencies across a plurality of OFDM symbols, and wherein resourceelements that are configured for ones of the first and second cells arebased at least in part on respective cell identifiers of the first andsecond cells; wherein the one or more first resource elements in the oneor more resource blocks used for the first TDOA-RS correspond to a firsttransmission instance, wherein the first and second TDOA-RS aretransmitted in a plurality of transmission instances; and wherein theplurality of transmission instances include one or more groups ofresource blocks in at least one subframe that are reserved for TDOA-RStransmission for at least the first and second cells, wherein the thirdcell transmits cell-specific reference signals and does not transmitTDOA-RS using the reserved one or more groups of resource blocks. 37.The apparatus of claim 36, wherein the third cell does not transmitcell-specific reference signals in a data region of MBSFN subframes. 38.The apparatus of claim 36, wherein the first and second TDOA-RS are nottransmitted using resource elements configured for cell-specificreference symbols.
 39. The apparatus of claim 36, wherein the apparatusis configured to report the determined time difference in response to atrigger that is based on radio resource control (RRC) signaling.
 40. Theapparatus of claim 36, wherein the apparatus is configured to determinea location of the apparatus based on the first and second TDOA-RS. 41.The apparatus of claim 36, wherein the first cell is a serving cell andthe second and third cells are neighbor cells.
 42. The apparatus ofclaim 36, wherein the apparatus is configured to determine a set of oneor more time differences of arrival for one or more neighbor cells fromwhich the apparatus determines a received TDOA-RS signal quality to beabove a particular threshold.